Luciana Maresca

Four-versus five-co-ordination in palladium(II) and platinum(II) complexes containing 2,9-dimethyl-1,10-phenanthroline (dmphen). Crystal structures of [PtCl2(dmphen)] and [Pt(η2-C2H4)Cl2(dmphen)]

The four-co-ordinate complexes with 2,9-dimethyl-1,10-phenanthroline [MX2(dmphen)](M = Pt, X2= Cl21, ClBr 2, Br23, or I24; M = Pd, X2= Cl25, ClBr 6, Br27 or I28) have been prepared for the first time. The crystal structure of [PtCl2(dmphen)]1, has been determined by X-ray diffraction methods. The complex has a square-planar geometry around the metal and the steric interaction between the methyl groups of dmphen and the cis chlorine ligands causes a narrowing of the Cl–Pt–Cl angle [85.8(1)°], a displacement of the two chlorine atoms from the N–Pt–N plane [0.286(4) and 0.372(4)A respectively], a bending of the phenanthroline (ca. 17°), and a rotation of the overall ligand plane with respect to the platinum co-ordination plane (ca. 28°). All interligand steric constraints are released in the five-co-ordinate complexes obtained from the four-co-ordinate species by direct uptake of olefins, [M(η2-olefin)X2(dmphen)](olefin = ethylene, a; propene, b; but-1-ene, c; cis-but-2-ene, d; trans-but-2-ene, e; or styrene, f). The structure of [Pt(η2-C2H4)Cl2(dmphen)]1a, has been determined by X-ray diffraction methods. The complex has a trigonal-bipyramidal geometry around the Pt atom; the phenanthroline ligand and the olefinic carbons occupy the equatorial plane while the two chlorine atoms are in axial positions. Bond distances and angles are similar to those found in other five-co-ordinate complexes of platinum(II). The rate of uptake of olefin by the four-co-ordinate complexes, the equilibrium constant (Kf) of the formation reaction [MX2(dmphen)]+ olefin ⇌[M(η2-olefin)X2(dmphen)], and the activation energy for olefin rotation in the five-co-ordinate complexes have been measured for different olefins and halogen ions. The uptake, in chloroform, takes place in a one-step process and the reaction rate increases by a factor of 104 going from the chloro 1 to the iodo 4 complex. The equilibrium constant for the formation reaction decreases by a factor of 103 going from ethylene to trans-but-2-ene and increases by a factor of 102 going from the chloro 1 to the iodo 4 species. The activation energy for olefin rotation (ethylene and propene)(ca. 20.5 kcal mol–1) is higher than any reported value for platinum complexes and does not appear to depend upon the nature of the halogen atoms.

Influence of steric and electronic factors in the stabilization of five-coordinate ethylene complexes of platinum(II): X-ray crystal structure of [PtCl2(2,9-dimethyl-1,10-phenanthroline-5,6-dione)]

Abstract Reactions of Zeise’s salt (K[Pt(η 2 -C 2 H 4 )Cl 3 ]) with oxidized phenanthroline ligands (1,10-phenanthroline-5,6-dione, phedon, and 2,9-dimethyl-1,10-phenanthroline-5,6-dione, Me 2 phedon) are reported. Comparison with analogous reactions involving unoxidized phen (1,10-phenanthroline) and Me 2 phen (2,9-dimethyl-1,10-phenanthroline) ligands indicates that these latter ligands are less capable to stabilize the five-coordinate species [PtCl 2 (η 2 -C 2 H 4 )(phenanthroline)] in which the phenanthroline and the olefin share the trigonal plane and two chlorines are in the axial positions. The X-ray structure of the four-coordinate species [PtCl 2 (Me 2 phedon)] indicates that the major difference between oxidized and unoxidized phenanthrolines is the loss of aromaticity of the central ring of phenanthroline. As a consequence, the oxidized phenanthroline becomes more flexible and can undergo a bow-like distortion so to reduce steric interaction between ortho substituents of phenanthroline and cis chlorine ligands. The increase in stability of the four-coordinate species with Me 2 phedon is concomitant with an increase in stability of the five-coordinate precursor complex with ethylene. In the latter case the stabilization is not of sterical origin but stems from reduced electron-donor properties of oxidized phenanthrolines. The balance of the two effects is such that the equilibrium between five- and four-coordinate species is more shifted in favour of the former species in the case of Me 2 phedon than in the case of Me 2 phen.

Five-Coordination in Platinum(II) and Palladium(II) Chemistry

Abstract In recent years a number of chelating ligands, when combined with a π-acid such as an alkene, have been found to stabilize five-coordination in palladium(II) and platinum(II) chemistry. The complexes have invariably a trigonal bipyramidal geometry with the bidentate ligand and the alkene in the equatorial plane. The π-acceptor capacity of the alkene and a small bite of the chelate ligand (between 70 and 85°) are both required for accommodating them in the equatorial plane where the d 2 z electron concentrations (which in the square planar d 8 complexes are located above and below the coordination plane) are also confined. Two mono-dentate ligands, in place of one chelate, do not stabilize the five-coordination since for entropic reasons the dissociation of one of them is strongly favored. A great bulk of the chelate ligand is also found to stabilize five-coordination since interligand steric interactions are smaller in the trigonal plane of a five-coordinate complex than in the square-plane of a ...

Modulation of properties in analogues of Zeise's anion on changing the ligand trans to ethene. X-Ray crystal structures of trans-[PtCl2(OH)(η2-C2H4)]− and trans-[PtCl2(η1-CH2NO2)(η2-C2H4)]−

To get further insight in the reaction of nucleophilic substitution upon changing the ligand trans to a η(2)-olefin, the reactivity of some monoanionic platinum(II) complexes (trans-[PtCl(2)X(η(2)-C(2)H(4))](-), X = Cl(-), 1, OH(-), 2, and CH(2)NO(2)(-), 3) towards pyridines with different steric hindrance (py, 4-Mepy, and 2,6-Me(2)py) has been tested. All crystallographic (2 and 3 reported for the first time) and spectroscopic data are in accord with a platinum-olefin interaction decreasing in the order 2 > 1 > 3, paralleling the decreasing electronegativity of the donor atom (O > Cl > C). Not only the platinum-olefin bond but also the bond between platinum and the ligand trans to the olefin appear to be strongest in 2 (Pt-O distance at the lower limit for this type of bond). In the reaction with py, the ligand trans to the olefin is displaced in 1 and 2. Moreover the reaction is in equilibrium in the case of sterically hindered 2,6-Me(2)py, the equilibrium being shifted moderately or prevalently toward the reagents in the case of 1 and 2, respectively. In the case of 3, the reaction with pyridines leads to substitution of the olefin instead of the carbanion. This is in accord with the observation that carbanions strongly weaken the trans Pt-olefin bond.

Cationic Complexes of Platinum(l1) Containing Olefins: A Type of Highly Electrophilic Substrate

Abstract The topic discussed in this article is the synthesis of cationic complexes of plati-num(I1) containing η2−olefins, their reaction with nucleophiles, and the cleavage, under acidic conditions, of the platinum-carbon bond of the addition productsobtained therefrom. Some general features related to the electrotl distribution and bonding, the structural and conformational preferences, the reaction mechanisms and reactivity are discussed.

Stable η- and σ-ethene cationic complexes of platinum(II)

Abstract Stable cationic complexes of platinum(II) with η- and σ-ethene have been obtained in the decomposition of five-co-ordinate [Pt(C2H4)Cl2(Me4en)] (1), (Me4en = N,N,N′,N′-tetramethylethylenediamine) in methanol. First a chloride ion dissociates forming the cationic intermediate [Pt(C2H4)Cl(Me4en)]+, (3); the two mol of (3) couple with a mol of diamine, via. nucleophilic attack on η-ethene, to give the bis σ-ethene dimeric cation [Me4en)ClPtC2H4N(CH3)2C2H4N(CH3)2C2H4PtCl(Me4en) 2+, (2). Complex (2) can be readily isolated at O°C as the stable perchlorate salt, and by reaction with stoicheiometric amounts of aqueous mineral acids regenerates the parent cation (3), which has also been isolated and characterized. The general validity of this stepwise decomposition mechanism for five-co-ordinate complexes of type (1), together with the reactivity of η-ethene in cationic platinum(II) complexes is discussed.

ISOMERISM OF AMIDES COORDINATED TO PLATINUM - X-RAY CRYSTAL STRUCTURE OF O-BONDED ACETAMIDE IN A PLATINUM(II) COMPLEX

O-bonded amide complexes of platinum(II), which are stable in acetone solution and which undergo solvolysis in neat water with a rate constant as slow as 8.7·10−5 s−1, are reported. The structure of a sample compound [Pt(Me5dien){O-MeC(O)NH2}](CF3SO3)2 has been solved by X-ray diffraction allowing comparison with related N-bonded (amide)platinum complexes. Isomerization from O-amide to N-amidate species takes place instantaneously under basic conditions. Protonation of the amidate species leads to the N-bonded amide. N-bonded amides, like the N-bonded amidates, represent the thermodynamically preferred species due to the higher affinity of platinum for nitrogen donors. In contrast O-bonded amides owe their stability to their kinetic inertness, where a high pH is required for deprotonation of the NH2 moiety, the leading step in the isomerization process. In the case of the 2-pyridone complex, the O-bonded form is unstable and isomerizes spontaneously to the N-bonded form, due to the higher acidity of the protonated pyridine-like nitrogen atom. Two rotamers are present in both the N-pyridone and N-pyridonate species. Hindered rotation stems from the steric rigidity of both rotating moieties [(Me5dien)Pt and pyridone/pyridonate].

Endocyclic versus exocyclic N-coordination to platinum(II) of some nitro-9-[(2-dialkylaminoethyl)amino]acridines

Abstract The kinetically controlled reaction products between the antitumor drugs 1-nitro-9-[(2-dialkylaminoethyl)amino]acridine (alkyl=Me, A 1 ; Et, A 2 ) or the inactive analogs 3-nitro-9-[(2-dialkylaminoethyl)amino]acridines (alkyl=Me, A 3 ; Et, A 4 ) and three different platinum(II) substrates have been investigated in chloroform and acetone solutions and found to be dependent upon the charge of the metal complex and the position of the tautomeric equilibrium (amino and imino forms) in the free acridine. Anionic ([Pt(η 2 -C 2 H 4 )Cl 3 ] − ) and neutral ([PtI 2 (DMSO)] 2 ) platinum substrates react with the dominant tautomer, either amino or imino, to give the endocyclic and exocyclic N-bonded derivatives, respectively. Positively charged substrates ([Pt(H 2 O)(Me 5 dien)] 2+ and [Pt(H 2 O)(dien)] 2+ ) coordinate exclusively to the exocyclic aminic nitrogens and this coordination mode stabilizes the imino form also in the case of 3-nitro acridines, which are present in both solvents as pure amino tautomers. For dien-platinum complexes fast exchange between free and coordinated acridine was observed at room temperature.

Dynamic behaviour of carbon-metallated palladium hydrazone complexes. Crystal structures of [{Pd[CH2CMe2C(N–NMePh)Me]Cl}2] and [{Pd[CH2C(N–NMePh)But]Cl}2]

The carbon-palladated species [{[graphic omitted]–NMePh)Me]Cl}2](1a) and [{[graphic omitted]MePh)But]Cl}2](2) have in the solid state a trans geometry and are planar; their crystal structures have been determined by X-ray methods. These species, as well as the bromo- and iodo-derivatives of (1a) and the complex [{[graphic omitted]Me2)But]Cl}2], exhibit temperature-dependent n.m.r. spectra which have been interpreted on the basis of cis–trans isomerization of the complexes and of hindered rotation about the N–N single bond of the hydrazone. The latter process is also present in the mononuclear species obtained from (1a) by bridge-splitting reactions with triphenylphosphine, pyridine, and acetylacetonate. Crystals of (1a) are triclinic, space group P, a= 10.642(8), b= 11.793(9), c= 6.269(6)A, α= 98.05(6), β= 97.46(6), γ= 111.91(7)°, and Z= 1. Crystals of (2) are monoclinic, space group P21/a, a= 24.059(12), b= 10.035(8), c= 6.018(5)A, β= 95.39(5)°, and Z= 2.

Five-co-ordinate ethylene complexes of platinum(II). Part 2. Synthesis and reactivity of some five-co-ordinate complexes of platinum with bis(hydrazones) and X-ray crystal structure of [butane-2,3-dione bis(methylhydrazone)]dichloro(η-ethylene)platinum(II)

The reaction of bis(hydrazones) with Zeises salt, K[Pt(C2H4)Cl3], to give stable five-co-ordinate complexes of general formula [Pt(C2H1)Cl2(L)] is reported [L = Ph(H)N·N:[graphic omitted]:N·N(H)Ph (L1), Ph(H)N·N:C(Me)·C(Me):N·N(H)Ph (L2), Ph(Me)N·N:C(Me)·C(Me):N·N(Me)Ph (L3), Me(H)N·N:C(Me)·C(Me):N·N(H)Me (L4), or Me2N·N:C(Me)·C(Me):N·NMe2(L5)]. These have a trigonal bipyramidal structure with the chlorine atoms in the apical positions, and the bidentate ligand (which co-ordinates through its α-di-imine unit) and the ethylene in the equatorial plane. The five-co-ordinate species decompose in solution releasing ethylene and giving four-co-ordinate complexes of formula [PtCl2(L)]. The rate of decomposition in 1,2-dichloroethane has been measured and its relationship to 1H n.m.r. data is discussed.The crystal and molecular structure of the five-co-ordinate [Pt(C2H4)Cl2(L4)] has been determined at room temperature from three-dimensional X-ray data collected by counter methods. The structure has been refined by full-matrix least-squares techniques to a final R(on F) of 0.035 based on 712 reflections. The title compound crystallizes in the orthorhombic space group Cmcm, with four molecules in a cell of dimensions a= 7.912(7). b= 15.821 (9), and c= 10.718(8)A. The co-ordination of the platinum atom is trigonal bipyramidal, assuming the ethylene molecule acts as a umdintate ligand. One symmetry plane contains the Cl–Pt–Cl group and bisects the bis(hydrazone) and the ethylene molecules, whilst the other contains the platinum atom and all the non-hydrogen atoms of the equatorial ligands. Significant bond distances are Pt–Cl = 2 304(3). Pt–N = 2.221(10), and Pt–C = 2.073(12)A. The results suggest that the equatorial and axial covalent radii of five-co-ordinate platinum(II) resemble those of three-co-ordinate platinum(0) and four-co ordinate platinum(II) respectively.